1. Field of the Invention
The present invention relates to the preparation of homogeneous copolyamides having both high melting points and good flexibility, and, more especially, to the preparation of such homogeneous copolyamides from hexamethylenediamine, adipic acid by itself or admixed with at least one other short-chain dicarboxylic acid, and a fatty acid dimer, the amount of short-chain diacid(s) relative to the total amount of the acids present [short-chain acid(s)+dimer acid] ranging from 50 to 99 mole %.
2. Description of the Prior Art
Copolyamides of the aforesaid type, based on hexamethylenediamine, adipic acid and dimer acid, are known to this art and are described in French Pat. No. 2,459,810, assigned to the assignee hereof. By "homogeneous copolyamide" there is intended a copolymer which forms only a single phase in the melt state, or a single amorphous phase in the solid state, as can be determined, for example, by scanning electron microscopy which reveals any dispersed phase which may be present via fixing with phosphotungstic acid.
A much simpler means for determining this homogeneity consists of observing the state of transparency of, on the one hand, a melt having a thickness of approximately 0.2 to 0.5 cm, heated at a temperature at least 10.degree. C. above its melting point and, on the other hand, a small-diameter rod or film obtained from this melt after rapid cooling, for example, by immersion in water at ambient temperature. An absolutely transparent rod or film indicates perfect homogeneity of the polymer; when the film is greater in thickness or when the cooling has been slower, a translucent zone may be the sign of some degree of crystallinity; a white film incorporating streaks or inclusions (fisheyes, for example) is the sign of a major heterogeneity. This homogeneity underlies the fact that the copolymers in question have thermomechanical properties which can be readily reproduced, in addition to good transparency. By a "copolymer having good flexibility" there is intended a copolyamide whose flexural modulus is significantly lower than that of the conventional polyamides such as polyhexamethyleneadipamide (nylon 66) or polycaprolactam (nylon 6) and is on the order of, or lower than, that of the polyamides considered to be flexible such as, for example, polyundecanamide (nylon 11).
In copolyamides of this known type, the crystallinity and, consequently, the ability to obtain high melting points and good thermomechanical properties are essentially contributed by the group of segments which are derived from the condensation of the short-chain acid molecules (adipic acid) with a part of the hexamethylenediamine molecules. The ability to have flexibility and good elasticity is substantially contributed by the group of segments which are derived from the condensation of the fatty acid dimer molecules with the other part of the hexamethylenediamine molecules. It is self-evident that, by changing the amount of short-chain acid relative to the total amount of the acids present in the range from 50 to 99 mole %, copolyamides are obtained whose melting points will be more or less high and whose flexibility will be greater or lesser. The copolyamides in which the amount of short-chain acid is in the range from 70 to 90 mole % are of particular interest because they have both high melting points and excellent flexibility.
Several specific processes which enable the preparation of such homogeneous copolyamides are also described in the aforenoted '810 French patent. According to a first process, the following steps are carried out:
(1) Prepolymers are prepared in a heterogeneous medium from a mixture of adipic acid, dimer acid and hexamethylenediamine, or the corresponding salts thereof, the amounts of the constituents of the starting mixture being such that the amount of adipic acid relative to the total acids is in the range from 50 to 99 mole % and that the contents of amino groups and of carboxylic groups do not differ by more than 5% as an absolute value, the polymerization reaction consisting of heating the starting mixture up to 270.degree. C. in a gradual and uniform manner over a period of time ranging from 30 minutes to several hours, at a water vapor pressure of from 1.3 to 2.5 MPa, which is the chemical equilibrium state corresponding to the start of the following phase (2);
(2) A homogenization phase is then carried out for a period of time of from 0.5 to 5 hours, at a temperature of from 270.degree. C. to 290.degree. C. under a water vapor pressure of from 1.3 to 2.5 MPa, during which the equilibrium state of the amidification reactions is not altered; and
(3) Lastly, a polycondensation is carried out such as to convert the homogeneous prepolymers thus obtained into the required copolyamides, the polycondensation reaction being carried out in a conventional manner at a temperature of from 260.degree. to 290.degree. C. at atmospheric pressure or at a lower pressure, for a period of time ranging from 30 minutes to several hours.
According to a second process, the following steps are carried out:
(1) Prepolymers are prepared in a heterogeneous medium from a mixture of adipic acid, dimer acid and hexamethylenediamine, or corresponding salts thereof, the amounts of the constituents of the starting mixture being such that the amount of adipic acid relative to the total acids is in the range from 50 to 99 mole % and that the contents of amino groups and of carboxylic groups differ by more than 5% as an absolute value, the prepolymerization reaction consisting of bringing the starting mixture to a temperature corresponding to the beginning of the following phase (2), in a gradual and uniform manner, over a period of time ranging from 30 minutes to several hours;
(2) A homogenization phase is then carried out for a period of time of from 10 minutes to 2 hours at a temperature of from 265.degree. to 290.degree. C.; and
(3) Lastly, the gradual addition of the deficient reactant is carried out in a conventional manner and the polycondensation is completed at temperatures of from 260.degree. to 290.degree. C. at atmospheric pressure or at a lower pressure for a period of time ranging from 30 minutes to several hours until the required copolyamides are obtained.
Lastly, according to a third process, copolymers are directly prepared at a temperature of from 150.degree. to 300.degree. C., in a homogeneous medium, from a mixture of adipic acid, dimer acid and hexamethylenediamine, the amount of adipic acid relative to the total acids being in the range from 50 to 99 mole % and the contents of amino groups and of carboxylic groups not differing by more than 5% as an absolute value, the medium being rendered homogeneous by the use of a third solvent for the mixture of the three starting materials or of the corresponding salts and/or the oligomers of adipic acid and hexamethylenediamine and dimer acid and hexamethylenediamine having a molecular weight of below 5,000, the said solvent being inert towards the amidification reactants and reactions.